Spike (M.I.) Walker
Desmid Algae (Micrasterias rotata)
English photomicrographer Spike (M.I.) Walker has been a consistent winner of the Nikon Small World competition for many years and has published many articles and a book about microscopy. Featured below is a photomicrograph of the desmid algae Micrasterias rotata, taken with darkfield illumination.
Organisms belonging to the genus Micrasterias are desmids, microscopic green algae that occur in all types of freshwater habitats around the world, usually in acid bogs or lakes. Only a few genera are found in alkaline waters. They thrive in waters that are deficient in nutrients, which is being threatened in many areas because of fertilizer run-off from residential and agricultural areas.
Desmids are characterized by extensive variation in cell shape and are typically one-celled, though sometimes filamentous or colonial. Each organism is divided symmetrically into two symmetrical halves, held together by an isthmus containing the nucleus. Each half of the cell has a single chloroplast. Micrasterias rotata is one of the largest desmids, and can occur as large as one-third of a millimeter.
The cell wall is composed of three layers impregnated with openings or pores and pectin spicules. Many species are phototropic and are capable of moving towards light by extruding a gelatinous substance through these pores. Desmids reproduce both sexually and asexually. Asexually, they reproduce by a simple division. Sexually, they reproduce either through conjugation, a means of exchanging nuclear material between two organisms, or by fusion -- when two organisms fuse to form a single new organism. Spores are rare.
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